The finding comes from Valerie Beral, MD, and colleagues with the U.K. Million Women Study.

They invited more than a million women living in Britain to take part in the study, starting in 1996.

Beral's team found that over a five-year period, women currently using hormone replacement therapy (HRT) for at least five years were 20% more likely to develop ovarian cancer and 23% more likely to die of ovarian cancer than women who had never used HRT.

Based on those findings, the researchers write that "since 1991, use of HRT has resulted in some 1,300 additional ovarian cancers and 1,000 additional deaths from the malignancy in the U.K."

However, the American Society for Reproductive Medicine notes that short-term HRT use probably isn't harmful for women using HRT to ease menopause symptoms.

HRT Study

When the study started, the women were about 57 years old, on average. They reported their current and past HRT use. Nearly two-thirds completed a follow-up HRT survey about three years later.

The researchers excluded women with a history of cancer other than nonmelanomaskin cancer and women who had had both ovaries surgically removed. That left more than 948,000 women.

Half of the women had ever used HRT. They include 30% who currently used HRT and 20% who had used HRT in the past.

The researchers followed the women for five years, on average. During that time, 2,273 women were diagnosed with ovarian cancer and 1,591 of those women died of ovarian cancer.